Recently, a number of very high strength-to-weight structures have been developed for special applications, such as aircraft and aerospace uses. Some of these structures, fabricated from metals such aluminum and titanium rely on very efficient configurations to obtain the desired strength, rigidity, etc. One of these configurations is the "isogrid" in which upstanding ribs generally integral with a sheet material are arranged as a series of substantially equilateral triangles. The performance of such a structure can be further improved by the addition of a narrow flange along the upper edge of the ribs, parallel to the base sheet, and a second flange at the rib base in the form of a thickened sheet strip along the rib. This provides ribs which are in effect I-beams integral with the sheet, which may form a skin for the structure. These may be called "flanged isogrid" structures. Isogrid structures may be further classified as "closed" where the grid is integral with a skin sheet and "open" where the skin is omitted. Examples of such a structure are provided in my copending U.S. patent application, Ser. No. 495,029, filed Aug. 5, 1974, and assigned to the assignee of this application (now U.S. Pat. No. 3.940,891), and my article entitled "The Isogrid," Machine Design, Apr. 19, 1973, pages 102-107. While these structures are very effective, they are still limited by the strength and stiffness of the metal in reasonable thicknesses. Further, weight becomes a problem if attempts are made to increase strength through thicker materials.
Another approach to achieving high strength-to-weight ratio structures has been through the use of "advanced composite materials" which use very high-strength fibers, e.g., epoxy and polyimide resins or aluminum. These materials have exceptional stiffness and strength and have been fabricated into a variety of structural components such as tubes, I-beams, etc. Fabricating more complex structures has been very difficult, due to the complex tooling required and the very precise forming and curing procedures required. For example, normal forming of sheets of these composite materials is impossible, since the fibers cannot be bent appreciably without fracturing. Because of these problems, advanced composite materials have been little used except for aerospace applications where high strength with low weight justify the very high cost of the materials and their fabrication.
Thus, there is a continuing need for structures with high strength and low weight.
It is, therefore, an object of this invention to provide a structure overcoming the above-noted problems.
Another object of this invention is to provide a structure of improved stiffness with a high strength-to-weight ratio.
Another object of this invention is to provide a structure combining the desirable features of isogrid structures and advanced composite materials.